Various systems for feeding aqueous chemicals under a vacuum exist. Commonly, these prior art systems utilize a venturi to create a vacuum. The vacuum draws a neat chemical through a calibrated glass tube and V-notch. The glass tube is utilized as an indication tool, wherein the V-notch is aligned with the calibration marks to control the flow rate through the metering device.
There are numerous drawbacks to these contemporary vacuum feeder systems relating to accuracy, operation and control. One drawback is that the calibration is determined for specific chemicals and their properties at standard conditions. Thus, if the chemical properties vary from those related to the calibration points the metered rate will be inaccurate, and these glass tube, V-notch devices will not indicate the inaccuracy. For example, changes in the chemical's specific gravity from that utilized for the calibration can result in significant metering inaccuracy.
Another drawback is when unstable chemicals, such as sodium hypochlorite for example, have gas breakout. The gas can become trapped in the V-notch device preventing proper operation of the metering device. Similar problems exist when debris or precipitates clog the ejector, or entrained gasses exist in the chemical feed stream. Again, the failure of the metering device will often go unnoticed until an operator visually checks the glass tube or a failure downstream in the fluid treatment system is noted.
Another drawback of the contemporary vacuum metering systems is the requirement for vacuum regulation systems. As the differential suction head changes, e.g. as the chemical level in the storage tank changes, chemical flow rate will change unless the differential head is addressed.